Organic materials, for example wood or paper, are generally combustible. This is also true, with a few exceptions, for the materials classed as plastics, i.e. materials based on synthetic or modified natural polymers.
Plastics are used in a wide variety of everyday applications. Plastics materials used in the construction and engineering sectors are in particular subject to specific requirements, which often include provision of adequate fire protection. The fire-protection criteria with which a plastic has to comply in a particular application are usually described in legislation, standards, and various other sets of regulations. Specific application-oriented fire-protection tests are used to demonstrate that a plastics material complies with the fire-protection requirements applicable to its application sector. Since plastics are generally combustible organic polymers, it is usually necessary to add flame retardant in order to pass the appropriate fire-protection tests.
There have hitherto been a variety of fire-protection specifications in the rail vehicle construction sector in Europe. The European Union has combined these by developing the European fire-protection standard EN 45545, ratified by CEN in March 2013; (the preliminary standard valid until that date having been CEN/TS 45545:2009). The German version was published as DIN EN 45545:2013 in August 2013.
All national standards relating to fire protection in rail vehicles have to be withdrawn by March 2016, and in future all plastics materials intended for use in rail vehicle construction have to comply with EN 45545:2013.
EN 45545 defines various product numbers: the product number covering cables for internal use is E1A, the product number covering cables for external use is E1B, and the product number covering flexible metal-rubber components, e.g. for vibration damping, is M1.
The nature and scope of fire-protection tests are defined in what are known as Requirement Sets applied to the various product numbers. By way of example, the tests for the abovementioned metal-rubber components with product number M1 are defined in Requirement Set R9 (R8 in CEN/TS 45545:2009).
The Requirement Sets moreover describe the results that have to be achieved in the prescribed fire tests in order to achieve classification in a particular hazard level. Hazard level 1 here represents the lowest level of requirements, while hazard level 3 is the most demanding level.
The hazard level achieved by a material or by the components produced therefrom then determines the specific design types and Operation Categories of vehicles in which it can be installed: if a component by way of example achieves hazard level 1 it can then be used only in design classes N, A, and D, and not S, for Operation Category 1 (and no other).
In contrast, components that achieve hazard level 3 can be used in all design classes and for all Operation Categories.
Vibration technology for rail vehicles is an application sector in which elastomeric materials are used. These likewise have to comply with the fire-protection requirements of DIN EN 45545:2013.
It is therefore of decisive importance for rail traffic that novel flame-retardant elastomer materials are found which comply with the requirements of the fire-protection standard DIN EN 45545:2013.
Flame retardants are usually dispersed homogeneously in the actual plastics material. However, this procedure is economically disadvantageous, because the flame retardants are primarily needed at the surface facing toward the seat of a fire—and are not needed, or are needed to a much lesser extent, in the interior of the plastics material. Another disadvantage resulting from homogeneous incorporation of flame retardants in plastics materials is that larger quantities, generally more than 10% by weight, of flame retardants are usually needed to achieve flame retardancy, and this substantially impairs the mechanical properties of the plastics system.
Another problem area also arises in the processing of elastomers, when comparison is made with thermoplastics, the forming process for which is a physical process: when elastomeric components are shaped it is desirable that a chemical reaction, termed vulcanization, takes place. Flame retardants that are incorporated into a compounded elastomer material can interfere in the vulcanization reaction with adverse results, and can impair processing properties or finished-part properties. They can also cause an undesirable reduction of the stability of what is known as the crude mixture in storage.
Provision of flame retardancy to a compounded elastomer material without coating, i.e. incorporation of all of the flame retardants into said material, therefore requires complicated experimentation, and the results will generally be applicable only to compounded materials of very similar type.
An approach that is significantly more efficient than homogeneous incorporation of a flame retardant is therefore application of a flame-retardant coating which localizes the flame retardants at locations where they are actually needed in the event of a fire. This approach can eliminate the disadvantages described above.
Flame-retardant coatings are likewise prior art. EP 2 196 492 discloses by way of example an elastomer body for vibration damping and springing, where the body comprises at least one layer of an elastic and flexible flame-retardant coating. The flame-retardant coating described in that document comprises expandable graphites as flame retardants.
Expandable graphites are intumescent flame retardant systems. These feature the ability to produce insulating barrier layers in the event of a fire. A plastics material coated with these materials can therefore be protected from the thermal decomposition due to a fire for a longer period of duration of the fire.
Expandable graphites or intumescent graphites are graphites that have been treated with strong acids and/or oxidants, for example sulfuric acid or potassium permanganate. The acids and/or oxidants here become intercalated between the layer planes of the graphite (intercalation), and thus disrupt the layer-lattice structure. On exposure to heat, the intercalated chemicals form gaseous products which force the individual carbon layers apart and lead to expansion of the individual graphite particles. The expansion volume here depends on the nature and quantity of the intercalated acid and of the oxidant, and in the event of a fire can be up to 400 times that of the original material.
The flame-retardant action of expandable graphites is based in essence on three effects. Firstly, expansion of the graphites consumes thermal energy, and thus cools the environment. Incombustible gases are moreover produced during expansion, and dilute the fire gases. Finally, the resultant insulation layers have high heat transfer resistance across a wide temperature range, insulating the material situated thereunder. Expandable graphites are among the most effective known flame retardants, and have already been used for more than 25 years in practical applications.
Expandable graphites are now used in a wide variety of application sectors, extending from coatings for steel beams and insulation-layer-forming systems for fire- and smoke-proof sealing of pipe ducts and cable ducts (EP 2 088 183 A1), and from foaming fire-protection tapes for security cabinets to flame-retardant foams for seats in aircraft or in rail vehicles (EP 2 260 066 A1).
EP 2 196 492 discloses an elastomer body for vibration damping and springing which comprises at least one layer of an elastic and flexible flame-retardant coating which comprises expandable graphites as flame retardants.
EP 2 196 492 also describes the results of fire-protection tests inter alia of CEN/TS 45545:2009, Requirement Set R8 (R9 in DIN EN 45545:2013). According to this Requirement Set, three fire-protection tests are carried out in accordance with the standards EN ISO 5659 (optical smoke density, smoke density, and toxicity) and, respectively, ISO 5660 (heat release rate).
The standard ISO 5660-1 describes a test relating to the fire behavior of construction materials where heat release rate is determined by the cone calorimeter method. The “average rate of heat emission” (ARHE) describes the average heat release rate, and is measured in kW/m2. MARHE (maximum average rate of heat emission) describes the maximal value of average heat release, likewise stated in kW/m2.
According to the prescribed requirement, for this abovementioned Requirement Set, the maximal permitted value determined in accordance with ISO 5660-1 for average heat release (MARHE) is at most 90 kW/m2 for hazard level 1 and 2, and at most 60 kW/m2 for hazard level 3.
The measured maximal value for average heat release (MARHE) stated for the flame-retardant-coated rubber described in EP 2 196 492 is 102 kW/m2 for an irradiation power level of 35 kW/m2. This flame-retardant-coated rubber does not therefore comply with Requirement Set R8 of CEN/TS 45545:2009, especially since the standard prescribes an irradiation power level of 50 kW/m2.
EP 2 196 492 lacks information revealing whether the abovementioned coated rubber complies with the requirements in respect of smoke density and smoke toxicity.
There is therefore an urgent necessity to develop novel, flame-retardant elastomer materials, or coatings therefor, with which it is possible to pass the fire tests in accordance with DIN EN 45545.